The present invention relates generally to a technique medical imaging and in particular to the field of digital X-ray. In digital X-ray imaging, two correction steps may be performed on raw images, namely offset correction and gain calibration. The gain calibration compensates for gain non-uniformities in the digital detector which may be present in an offset corrected raw image using a gain calibration map. Specifically, the present invention relates to a process for adapting a gain calibration map in an X-ray image to reduce or eliminate artifacts, particularly those visible in the display of a flat object.
Digital X-ray imaging systems are becoming increasingly widespread for producing digital data which can be reconstructed into useful radiographic images. In current digital X-ray imaging systems, radiation from a source is directed toward a subject, typically a patient in a medical diagnostic application. A portion of the radiation passes through the patient and impacts a detector. The surface of the detector converts the radiation to light photons which are sensed. The detector is divided into a matrix of discrete picture elements or pixels, and encodes output signals based upon the quantity or intensity of the radiation impacting each pixel region. Because the radiation intensity is altered as the radiation passes through the patient, the images reconstructed based upon the output signals provide a projection of the patient's tissues similar to those available through conventional photographic film techniques.
Digital X-ray imaging systems are particularly useful due to their ability to collect digital data which can be reconstructed into the images required by radiologists and diagnosing physicians, and stored digitally or archived until needed. The digital data produced by direct digital X-ray systems can be processed and enhanced, stored, transmitted via networks, and used to reconstruct images which can be displayed on monitors and other soft copy displays at any desired location.
Artifacts may be introduced into the generated X-ray image due to various factors in the imaging chain. These factors include non-uniformity in scintillator thickness, variations in read-out electronics, tube flux distribution, and inhomogeneities in the cover material overlaying the scintillator. To correct image artifacts arising from these various factors, a gain calibration process employing a gain compensation map may be performed. Because some of these factors, such as contrast variations and artifacts related to scintillator thickness and cover material inhomogeneities, are strongly dependent on the X-ray spectrum incident on the detector, the gain compensation map may be a function of the x-ray spectrum incident on the detector.
Gain calibration is typically performed during installation or system maintenance. The calibration, if fully accounting for variables in the system, would be based upon a number of conditions or combinations of settings, such as the source settings, radiation filter settings, thickness and composition of attenuating object and so forth, which affect the spectra of the radiation used to produce images. The amount of time available to perform this calibration is typically limited however, and, in the interest of time, the digital detector is typically only calibrated at finite acquisition conditions, typically two in the specific case of digital mammography. One of these finite number of calibration maps is then applied to any image acquired at the diagnostic spectrum of interest during actual use. In addition, the patient or other imaged object provides some finite filtration of the spectra which depends on the thickness and composition of the imaged region. The use of a finite number of gain calibration maps in combination with the wide range of spectra actually present in routine imaging situations often produces visible artifacts, particularly in regard to those factors which vary in a spectrum dependent manner.
These deviations and deficiencies in the gain correction maps are particularly notable or evident when the imaged object or region is flat relative to the detector, such as in mammography systems. In mammography, as well as certain other diagnostic situations, the image area of diagnostic interest extends to the edge of the detector. However, due to non-uniformities at the edge of the detector, gain correction may be compromised near the edge, giving rise to strong spectrum sensitive artifacts at the edge. In particular, there may be a fall off in the thickness of the scintillator material, typically cesium iodide (CsI), which can result in spectrally-sensitive contrast variations or artifacts. In such situations, use of a finite number of gain correction maps, each acquired at discrete operating points, may be inadequate to properly correct the gain near the detector edge due to the variation in scintillator thickness and the spectrally-sensitive nature of these artifacts.
In addition, the spectrum dependent variations in gain may result in otherwise acceptable detectors being rejected during quality assurance when a test image is acquired at a spectrum other than those used for calibration. In particular, detector testing and quality assurance for mammography systems, or for other systems where good performance to the edge of the detector is desired, may reject detectors which are otherwise acceptable in these circumstances. In particular, while such variations may not affect the quality of most images, they may nevertheless cause a residual error after gain correction which is higher than allowed to pass image quality specifications. Moreover, calibration at X-ray spectra different than the quality test or diagnostic spectra may lead to detector failure even though proper gain calibration at those spectra would result in an acceptable detector. Poor detector yields from the manufacturing processes result even though proper calibration would improve such yields. A technique for providing gain correction in a time efficient manner and which allows for the artifact-free flat field imaging for all possible acquisition conditions is therefore desirable.